Stimulation of cardiac tissue using a leadless cardiac stimulation system has been disclosed earlier by the applicant. Generally, such a system comprises an arrangement of one or more acoustic transducers, and associated circuitry, referred to as a controller-transmitter, and one or more implanted receiver-stimulator devices. The controller-transmitter generates and transmits acoustic energy, which is received by the receiver-stimulator, and the receiver-stimulator in turn converts the acoustic energy into electrical energy, which is delivered to the tissue through electrodes.
The controller-transmitter may be externally coupled to the patient's skin, but will usually be implanted, requiring that the controller-transmitter have a reasonable size, similar to that of implantable pacemakers, and that the controller-transmitter be capable of operating for a lengthy period, typically three or more years, using batteries. The small size and long operational period require that the system efficiently utilize the acoustic energy from the controller-transmitter with minimal dissipation or dispersion of the transmitted energy and efficient conversion of the energy by the receiver-stimulator.
Charych (U.S. Pat. No. 6,798,716) describes various strategies for locating an acoustic receiver. Charych describes methods for charging wireless devices (receivers) from a controller-transmitter that is powered through a plug, providing power in excess of 1000 W. In contrast, a leadless cardiac stimulation system, where the power flow is 6 orders of magnitude lower, requires completely different methods and systems for locating the receiver, which are not described by Charych.
Briefly, in its simplest form, the receiver-stimulator comprises one or more acoustic piezoelectric receiver elements, one or more rectifier circuits, and electrodes. The piezoelectric receiver elements couple power from the acoustic field generated by the controller-transmitter and convert it into electric power. If applied directly to the tissue this AC electrical power does not stimulate the tissue because its frequency is too high for excitation/stimulation. In order to initiate a paced heart beat, or provide other therapeutic stimulation to tissues, the rectifier circuits convert all or some of the available AC electrical power to an electrical pulse that is applied to the cardiac tissue through the electrodes. The acoustic field is generated and transmitted either by an externally placed or an implantable controller-transmitter that is remote from the location of the receiver-stimulator.
The acoustic energy generated by the controller-transmitter is generally referred to as an acoustic beam or ultrasound beam and is characterized by acoustic intensity (I) measured in Watts/square meter. In order to create an acoustic intensity of Io over an area Ao the controller-transmitter must expend at least Io*Ao Watts of power. Only the portion of this acoustic beam that intersects the receiver-stimulator will be available as electrical power. If the area Ao is larger than the cross sectional area or aperture of the receiver Ar, then the ratio Ar/Ao represents that fraction of the power in the acoustic beam that is available to the receiver-stimulator. Therefore the optimally efficient acoustic beam is very narrow and only intersects the receiver elements of the receiver-stimulator.
The controller-transmitter has one or more piezoelectric transducers that convert electrical power into acoustic power creating the acoustic beam that is directed at the receiver-stimulator. The ability of the controller-transmitter to generate this acoustic beam over a small area is characterized by its focal or directivity gain. In general the larger the cross sectional area (referred to as the aperture) of the controller-transmitter transducers, the higher the directivity gain will be. This requires the controller-transmitter to have a wide aperture transmitter that focuses acoustic energy at the receiver-stimulator. It also requires the controller-transmitter to steer or direct the acoustic beam at the receiver-stimulator. This can be accomplished by using a phased array that uses beam-forming techniques to steer the acoustic beam at the receiver-stimulator. Steering can be accomplished by adjusting the phases and amplitudes of the electrical drive signals to the transducer array, which results in adjusting the direction and focal distance of the transmitted beam.
If the location of the receiver-stimulator or the controller-transmitter does not change over time, the controller-transmitter could be configured at the time of implant to optimally select a focused beam profile that is aimed at the receiver-stimulator location determined at the implantation time. However, in the case of the leadless system, the receiver-stimulator can be expected to move due to cardiac motion, breathing, or body orientation. Moreover, the controller-transmitter may move slightly due to body orientation or body movements or migration. Therefore, to accommodate the movement of the controller-transmitter and the receiver-stimulator, inventors herein have realized that successful operation in the simplest implementation would require a relatively broad beam acoustic emission. However, in this mode of operation most of the transmitted acoustic energy may pass by the receiver-stimulator and not used efficiently. Hence, inventors herein have further realized that to improve efficiency the transmit beam needs to be significantly sharpened or focused, and reliable operation would require continuous, specific knowledge of the location of the receiver-stimulator.
For the above reasons, it would be desirable to provide a leadless system that efficiently transmits and receives acoustic energy. It would also be desirable for the transmitted beam to be adjusted, to be as focused as possible at targeting the receiving element(s) of the receiver-stimulator. It would be particularly desirable if the location of the receiver-stimulator is known to the controller-transmitter, and, thereby, a focused acoustic beam could be aimed and transmitted toward the receiver-stimulator. It would also be desirable if the receiver-stimulator is located using mechanisms that minimize the size and complexity of the receiver-stimulator such that additional circuitry or energy consumption is not imposed upon the receiver-stimulator.